In general, two types of artificial heart valves are used to replace defective heart valves: mechanical valves and tissue valves. Although implantation of artificial heart valves has traditionally occurred through open heart surgery, research and experimentation are being done to develop valves that can be placed in a patient percutaneously, thereby avoiding open heart surgery.
Implantation of mechanical valves, which are durable, requires open heart surgery, risks peri-valvular leakage on the outside of the valve between the valve and the attachment wall, and requires a lifetime of administration of anti-coagulants, which requires close (usually bi-weekly) monitoring in order to avoid either bleeding or thrombotic/embolic stroke. Mechanical valves also risk development of stenosis at the valve replacement site, and incur chronic hemolysis (damage to red blood cells by the mechanical action of the valve).
Tissue valves typically last from 10 to 15 years in less active and elderly adults and are of porcine or human origin. They fail because the tissue of the valve begins to wear, at least in part because the valves are retrieved after already having undergone partial lifetimes of use. Tissue valves in younger people wear out more quickly due to the more active blood flow in younger people, which causes rapid calcification and places great mechanical demands on the valves. The risk of death or serious complications from surgical valve replacement is typically from 1% to 5% depending on the health of the patient and the skill of the surgeon. Therefore, it is preferred that a valve only be replaced one time.
Mechanical valves last longer in younger patients because the patients are still growing. However, pediatric valve replacements are particularly challenging because the patients frequently outgrow the implanted mechanical valve and require surgical intervention to replace the pediatric valve with a larger valve.
Progressive deterioration of a tissue valve can lead to stenosis, which manifests itself as an obstruction of forward flow through the valve when the valve is in its open position. More commonly, deterioration of a valve produces tears in the valve leaflets that cause regurgitation, which manifests itself as a leakage in the valve when the valve is in its closed position.
Known synthetic valves, although configured to mimic native valves, never assimilate fully into the surrounding tissue following implantation. In addition, attachment of known synthetic valves is accomplished using a ring that remains in a single plane following implantation, thereby risking perivalvular leakage in the same manner as the attachments of mechanical valves.
The tricuspid valve separates the right atrium from the right ventricle, and the mitral valve separates the left atrium from the left ventricle. The annuluses in which these valves are mounted typically comprise dense fibrous rings that are attached either directly or indirectly to the atrial and ventricular muscle fibers. In a valve replacement operation, the damaged leaflets are excised and the annulus is sculpted to receive a replacement valve. Ideally, the annulus presents relatively healthy tissue which can be formed by a surgeon into a substantially uniform ledge that projects into the opening created after a native valve is removed. The time and spatial constraints imposed by surgery, however, often dictate that the shape of the resulting annulus is less than perfect for attachment of a sewing ring. Moreover, the leaflets of the valve and the annulus may be calcified, and complete annular debridement, or removal of the hardened tissue, can result in a larger opening and a more gradually sloped annulus ledge for attachment of the sewing ring. In short, the contours of the resulting annulus vary widely after the natural valve has been excised.
Conventional placement of a valve is intra-annular, with a valve body deep within the narrowest portion of the annulus to enhance any seal effected by the sewing ring/suture combination and reduce the chance of perivalvular leakage. Surgeons report using at least 30 simple sutures or 20 mattress-type sutures to prevent leakage.
The implantation of a prosthetic heart valve, including mechanical valves and bioprosthetic valves (i.e., “tissue” valve), requires a great deal of skill and concentration given the delicate nature of the native heart tissue, the spatial constraints of the surgical field and the criticality of achieving a secure and reliable implantation. It is of equal importance that the valve have characteristics that promote a long valve life and have minimal impact on the physiological makeup of the heart environment.
Given the uneven nature of the annuluses, the design of the sewing ring and the method by which the sewing ring is fixed into place are perhaps the most crucial aspects of prosthetic heart valve implantation. Due to the inability of conventional sewing rings to easily stretch, if the selected size of the sewing ring is even slightly too small, attachment can only be achieved by placing undue tension on the tissue and sutures. As a result, a great deal of care and accuracy by the surgeon is needed in the selection of a valve size that precisely matches the valve annulus of the patient. Unfortunately, standard sizing tools are provided in increments based on an overall opening size, and may not be able to accurately measure a less than optimally formed annulus. The surgeon thus must select an approximate valve size.
Accordingly, there is a need in the art of valve replacement procedures for a valve having the benefits of a tissue valve and the longevity of a mechanical valve, without the side effects or disadvantages of either. Surgical outcomes would also benefit greatly by an improved sewing ring, permitting improved tissue attachment in all valve replacements.